1. Field of the Invention
The invention is related to an improved switching arrangement. The invention is particularly but not exclusively directed to an efficient method and apparatus for providing multiple supply voltages from a single voltage source for a switched-mode voltage supply.
2. Description of the Related Art
Envelope tracking is a well-known technique for improving the efficiency of RF (radio frequency) power amplifiers that handle a non-constant envelope signal. In order to implement envelope tracking techniques there is a need for efficient power modulators that can follow a rapidly changing signal. A problem is to provide an accurate fit to a rapidly changing signal whilst retaining the efficiency of a switched converter.
It is well-know to those skilled in the art that a much closer fit to a switched signal can be achieved by switching between a multiple level signal than can be achieved between a single level and ground. This can be understood with reference to FIGS. 1(a) and 1(b).
In FIG. 1(a) the desired output signal waveform 2 is approximated by a single switched level signal 4. This results in errors as denoted by the shaded areas 6. In FIG. 1(b) a multiple (in this example three) level switching element is provided, such that the switched level signal can be switched between more than one level in addition to ground. As seen in FIG. 1(b), the multiple switched level signal 8 more closely approximates the desired output signal waveform 2, such that the errors as denoted by shaded areas 10 are much smaller.
A simple example of an arrangement for providing a multiple switched level signal is shown in FIG. 2. A plurality of voltage sources 12, 14, 16, having nominal values V1, V2, and V3 respectively, are connected between electrical ground and distinct terminals 18, 20 and 22 of a multi-pole switch 30. An additional terminal 24 of the switch is also connected directly to electrical ground. The switch is controlled to connect one of the voltage sources, or electrical ground, to a further switch terminal 26, which is connected to one terminal of a load 28, having another terminal connected to electrical ground. In this way one of four voltage supply levels (or ground) is provided to the load, to provide an output signal as illustrated in FIG. 1(b).
However, in certain implementations, such as in portable equipment, only one voltage source is provided from a single battery. Therefore to achieve a multiple switched level signal, a suitable converter has to be provided to convert the battery voltage to the desired multiple voltage levels. This is achieved, as known in the art, by using a buck-boost converter. A particularly advantageous arrangement of a buck-boost converter is disclosed in United Kingdom Patent Application Publication No. 2460072, in the name of Nujira Limited.
FIG. 3 illustrates the simplified boost (not buck) principle of an advantageous voltage converter as disclosed in United Kingdom Patent Application Publication No. 2460072. The voltage converter is generally designated by reference numeral 32, and includes a switch 34. Additionally illustrated is a further modulator switch 57, which comprises a voltage modulator for selecting a currently desired power supply level.
The converter 32 of FIG. 3 provides three different power supply voltages V1, V2, V3 on lines 36, 38, 40 from a single voltage source provided by a battery 41. In addition an electrical ground signal level is provided on line 42. An inductor 44 is connected between one terminal of the battery 41 and a terminal of the switch 34. The switch 34 is controlled to selectively connect the inductor to any one of lines 36, 38, 40, or to ground on line 46. A plurality of capacitors, corresponding to the number of voltage signals being generated, is provided, each connected between an output signal line from the switch 34 and ground. Thus there is provided capacitors 48, 50, and 52, respectively connected between the lines 36, 38 and 40 and electrical ground. The switch 34 switches the output of the inductor 44 to the lines 36, 38 and 40, and hence the capacitors 48, 50, and 52, in such a way that the voltages V1, V2, and V3 are formed on the lines 36, 38, and 40. The modulator switch 57 then switches a selected one of the voltages V1, V2, V3 to an output, by selecting one of the lines 36, 38, 40, and 42 to be connected to an output line 56 and connected to a further inductor 58, the other terminal of the inductor 58 providing the modulator output on line 60. The switch 34 typically switches at a lower rate than the switch 57, because charge is stored in the capacitors.
With the arrangement of FIG. 3, effectively two voltage converters are connected in cascade. In addition, each switch(in the switch bank 34 and the switch bank 57) has a resistive loss associated with it that is inversely proportional to its size. Since current is always flowing through one switch in each of the switch banks 34 and 36 at any instant in time, the effect of the switch banks is as if the current flows permanently through a fixed resistor in each of the switch banks.
Because of the cascading of the switch banks, and because there is half the number of switch transistors in a buck converter in the front end, then for an equivalent unit resistance the transistors and their associated metal interconnects have to occupy a much larger area. When the routing is applied on an IC (integrated circuit), this becomes a serious problem because of the relatively high resistance of the thin metal. The critical interconnect is illustrated in FIG. 3 by the drawing of thick lines.
It is thus an aim of the invention to provide an improved arrangement for a wideband switched mode power supply based upon a switching between multiple voltage levels.